Abstract

Past research on rill erosion has mainly focused on hydraulic conditions of overland flow. Because very few studies addressed the effect of soil physical properties on rill erosion so far, this study puts emphasis on the impact of soil bulk density on rill erosion. In order to generate rill erosion under controlled and variable conditions a 2 meter long and 0.1 meter wide flume was used and adjusted to 2% slope. The flumes discharge was raised stepwise from 0.060 l*s-1 (Q1) to 0.125 l*s-1 (Q2) to 0.300 l*s-1 (Q3). Homogenized silty to loamy soil material was filled in the flume and a plane bed prepared. At the beginning of the experiments runoff formed a nearly uniform water sheet and initial sheet erosion acted on the soil surface. During this stage microrills formed and selective sediment transport conditions provoked the formation of ripples. Ripples were found to be the precondition for the development of headcuts. Once headcuts had formed they migrated uphill leaving a rill, in which runoff concentrated further. Hydraulic conditions shifted from sheet to rill flow causing sediment concentrations to rise. Soil bulk density and soil texture control the formation of ripples and thus the onset of rill erosion. The headcut formation rate and headcut duration, as well as the rate of headcut retreat, were found to be affected by soil bulk density. The relationships are non-linear and discontinuous. The headcut formation rate, which expresses the number of headcuts per time and slope length unit influences directly the sediment concentration of surface runoff and thus soil loss by rill erosion.